The consumer champion carried out 28 comparisons between booking direct and using OTAs eDreams, Gotogate, Kiwi and Opodo. It found in all cases it was cheaper to book extras with the carriers despite the OTA headline prices often being “marginally cheaper”.

Which? found 60% of those who had used a third party site to book were unaware they could be charged more for luggage than by the airline.

Which? was quoted £556 by eDreams for a return from Gatwick to Orlando with British Airways, £2 less than if booked with BA direct. However, once Which? added hold luggage and seat selection, the price increased by £258 to £814. The same flight and extras booked directly with BA was £712.

Rory Boland, Which? Travel editor, said: “It’s easy to be tempted by headline airfares from online travel agents that save a few pounds but you are always better off booking directly with the airline. Not only is it likely to work out cheaper in the end, but in the event that something should go wrong, it will be clear where the responsibility lies.

“If you’re looking to find the best value prices, flight comparison sites will always be the preferable option for finding the cheapest seats. Likewise, if you’re keen to book as a package and add accommodation to your flight booking, Which? would always advise using a reputable package holiday firm.” 

The magazine also found a Ryanair flight from Stansted to Athens priced at £104 via Opodo, £3 cheaper than Ryanair. Once researchers added one 20kg hold suitcase, a 10kg cabin bag with priority boarding, and chose a seat, the price more than doubled to £261. Booking with Ryanair and making the same selections was £66 less. 

Ryanair said: “OTAs have no commercial agreements in place to sell our flights.  OTAs clearly breach the Terms of Use of the Ryanair website by using screen scraper technology to scrape our website and mis-sell our flights.”

The magazine also criticised the level of customer service provided by OTAs, with many charging extra. It said Kiwi.com charged around £17pp to access higher caller priority, email support and other perks such as free trip changes. 

Which? warned: “If you choose not to pay, you’ll be low priority when calling, and could face additional charges of up to £30 for any additional services, compared to £10 if opting for the ‘plus services’ option.”

OTAs were also blasted for offering AirHelp Plus, a £9 service advertised as ensuring compensation in the event a flight is delayed or cancelled. Which? said EU261 compensation rights meant there was “no need to pay any kind of fee”.

It singled out Trip.com for criticism in this regard: “The way Trip.com presents AirHelp’s service suggests that passengers wouldn’t get their compensation without it – something which is untrue and could potentially be unlawful under the Consumer Protection from Unfair Trading Regulations 2008.” 

Trip.com said: “The display of this service is in line with our responsibilities under UK consumer law.”